1. Field of the Invention
The present invention relates to a method for heat treating steel or more precisely, a method for reducing the grain size of steel and precipitating carbonitrides in steel to thereby increase the strength of the steel.
2. Discussion of the Background
Various types of heat treatments for improving the strength of steel are known, including, for example, carbonitridation hardening, induction hardening, quenching, tempering, etc. In general, hot-rolled or cold-rolled carbon steel or middle- or low-alloy steel, or hot-forged steel is cut, shaped, and subjected to any of these heat treatments in which the steel is heated up into the austenitic range, thereby increasing the hardness of the steel surface or the entire steel, and increasing the strength of the thus heat-treated steel.
However, the original austenitic grain size of steel as treated according to the conventional heat treatment noted above could be only No. 8 or so in terms of the JIS steel grain size (Gc). Therefore, the conventional heat-treated steel does not always exhibit satisfactorily increased strength when used in transmission gears or the like, where it is exposed to severe environments that require high fatigue strength, pitting strength and impact strength.
In examined Japanese Patent Application No. hei 7-13294, there is disclosed a carbonitriding method for heat treatment reducing of steel for reducing the grain size of the steel to thereby increase the strength of the steel. The method comprises a carburization hardening step of heating the steel, which includes chromium, in an atmosphere containing a carburizing gas, cooling it down under the A, transformation point of the steel and quenching it, followed by a second hardening step of heating up the thus-carburized steel between 850 and 900.degree. C. so as to reduce the grain size of the steel and resolve chromium carbide, treating carbonitriding to educe carbonitrides on the outer layer of the steel in an atmosphere between 800 and 850.degree. C. and there after again quenching. However, in this method, there is no description about cooling speed from the beginning to the end of the cooling down of the steel during the carburization hardening step. If the cooling speed is too slow, sufficient martensitic formation in the outer layer of the steel is not obtained. Therefore, the steel is not sufficiently increased in strength because the grain size of the steel is not sufficient reduced and there is only a small amount of enduced carbonitrides.